I'd make some gunpowder (charcoal, bovine poo and a couple of other easily found ingredients) and become a self-proclaimed god (with special effects and stuff).

I'd make some gunpowder (charcoal, bovine poo and a couple of other easily found ingredients) and become a self-proclaimed god (with special effects and stuff). Then I'd teach my minions Klingon and we'd all be using a single language (with less than 6k words) today.